1. Field of the Invention
The present invention relates to an apparatus for receiving a television signal which was subjected to a band compressing process and, more particularly, to an apparatus for receiving a television signal in which a signal band width is reduced by the sub-Nyquist sampling between frames and an interpolation control signal between fields to control whether the offset sub sampling interpolating process between fields is executed or not is transmitted together with an image signal.
2. Description of the Related Art
A system for reducing a signal band width by the sub-Nyquist sampling when transmitting a wide band image signal such as a high definition television signal has been proposed by "HD-TV Broadcasting System Using Single Channel Satellite (MUSE: Multiple Sub-Nyquist Sampling Encoding)", Television Society Technical Report, TEBS 95-2, from Nippon Hoso Kyokai (Japan Broadcasting Corporation). Such a system is called a movement correcting type multiplex sub-Nyquist sampling transmission system. In detail, offsets are added to the sampling phases between fields and between frames and processes are executed such that the sampling phases are returned to the original phases by four fields, thereby transmitting an image signal. On the reception side, a frame memory is provided and the transmitted image signal is stored into the frame memory, the signals of four fields which are successively transmitted are synthesized to reconstruct one television image as a fundamental method. However, in the television for performing the interlace scanning of 2:1, the offset sub-sampling process between frames and the reconstructing process of the signal are the fundamental processes in the signal processes for every field. In a receiver of such a system, the interpolating processes between frames and between fields are executed for the image signals which were sub-sampled and transmitted, thereby reproducing an image. However, with respect to the portion accompanied with the motion in the screen, when such an interpolating method using past sampling points also is used, multilines become dim or a sampling pattern becomes a dotted pattern and appears on the display screen. Therefore, it is necessary to reproduce an image by using only the sampling points of the present field. Since the interpolating methods of an image to the still area and animation area differ, the interpolating methods are switched by using a control signal which is transmitted from the transmission side together with an image signal. The control signal is called a field interpolation control signal and is a binary signal of one bit which is output as a control signal to indicate whether the field interpolating process is executed or not for the field from the transmission side.
A conventional example regarding the present invention will now be described hereinbelow with reference to the drawings. FIG. 2 shows an example of a part of a conventional receiving apparatus of a television signal which was subjected to a band compressing process. An analog image signal is transmitted from the transmission side and converted into a digital video signal by an A/D converter. The digital video signal is supplied to a signal input terminal 1. The input digital video signal is supplied to both of an animation image system processing circuit 2 to perform a signal process of an animation area of an image and a still image system processing circuit 3 to perform a signal process of a still area, respectively. The signal which was delayed by a time of one or two frames by using delay means such as a frame memory or the like and a signal of the current frame are input to a movement amount detecting circuit 4. The difference between the two input signals is calculated by the movement amount detecting circuit 4, so that a movement amount is obtained. The difference signal is used as a control signal and output signals of the processing circuits 2 and 3 are mixed on a pixel unit basis at a ratio corresponding to the movement amount by a mixing circuit 5. The signal mixed by the mixing circuit 5 through the still and animation image system processing circuits is now the signal which was subjected to only an interpolating process between frames. Therefore, the mixed signal is then subjected to an interpolating process between fields by a field interpolating circuit 8. In the field interpolating circuit 8, the interpolating methods for the still area and animation area are different. For the still area, what is called the interpolation between fields is executed by using sampling points of two adjacent lines before one field which were time delayed by using field memories or the like and a sample point of the current line. For the animation area, since a movement exists between fields, the sample point before one field cannot be used. Therefore, what is called the in-field interpolation is executed by using only the sampling point of the current field. The interpolated signal is supplied to a signal output terminal 11. The switching control between the process for the still area and the process for the animation area is executed by using the OR of an output signal of a binarizing circuit 6 which receives an output signal of the movement amount detecting circuit 4 and a field interpolation control signal which is transmitted from the transmission side and supplied to a signal input terminal 7. The receiving apparatus of the television signal which was subjected to a band compressing process as mentioned as the conventional example uses a technique such that the image signal which was band compressed by using the 4:1 sampling method having a sampling pattern as shown in FIG. 3 is reconstructed by using the in-field interpolation, interpolation between frames, and interpolation between fields. To reconstruct the still area of an image, the interpolating process is executed by using the sampling points of four fields. A transmittable spatial frequency area as shown in FIG. 4A is provided. To reconstruct the animation area, since a movement exists between fields, the interpolation is executed by using only the sampling points of the field which are always transmitted. A transmittable spatial frequency area as shown in FIG. 4B is provided. It will be understood from the diagram that with respect to the still area, the resolution in the oblique direction slightly deteriorates as compared with the case where the square lattice-shaped sampling was executed. However, a degree of deterioration is not so large and no problem will occur when considering the reduction of the visual acuity of human being in the oblique direction. On the other hand, with regard to the animation area, the interpolation is executed by using only the sampling points of the current field, so that the spatial frequency area is fairly narrower than that of the still area and the 2:1 interlacing process is executed. Therefore, the resolutions in the vertical, horizontal, and oblique directions deteriorate to 1/2 of the resolutions of the still area, respectively. Thus, there is a problem such that there occurs a case where the user feels unnaturality of the picture quality due to the difference between the resolutions in the still and animation areas.